The invention relates to gate drive circuits and, in particular to such circuits with reduced electromagnetic interference (EMI) emission without sacrificing efficiency as compared to conventional circuits, such as those requiring large gate resistor insertion.
In general, switching applications have a trade off between efficiency and EMI emissions. In order to get better efficiency, higher switching speed is required. The higher the speed, the steeper the transient which is generated due to stray inductances and capacitances and the greater the EMI.
An aim of this invention is to reduce EMI emission without reducing efficiency significantly.
In a typical hard switching application, one of the main noise sources of EMI emission is di/dt spikes caused by the body diode reverse recovery charge QRR which usually has sharp waveforms and triggers harmful ringing. This Qrr discharging current can be sharp and large because of low impedance between the power supply top rail and bottom rail and high resonant factor Q.
One of the simplest conventional methods for reducing EMI noise in gate driving stages is to place a resistor Rg in series with the gate of the MOSFET, slowing down turn-on/turn-off speed by limiting the amount of gate drive current.
One problem with this method is the effectiveness of this gate resistor is not equal to either the Qrr discharging period or transition period. The effectiveness of Rg varies during the turning on process due to the Millar effect; the resistor slows down the turn-on process the most when the drain voltage is in transition. However, the time period when di/dt needs to be reduced for smaller Qrr is before the drain-source voltage starts to fall. As a result, the amount of resistance which is required to slow down the process has more effect in the reverse recovery discharging period than in the voltage transition period. Therefore switching loss increases significantly even though reverse recovery discharging is slowed down.